If Geoff Wilson ever realises his ambition to become a rat-catcher, he says he will have his local Connexions office in Brighton to thank. Yet, as the prospects grow bleaker for young job seekers, Connexions – the careers and personal advice service – itself faces decimation because of government spending cuts.

Connexions has become an easy target, says Unison, the local government workers' union, and vulnerable young people who can slip through the net at school will suffer most. Often they seek help with social problems of all kinds, as well as career and course guidance.

Local authorities, which fund Connexions, are struggling to digest a 24% area grant reduction announced in June. The service's problems are acute because much of its funding for 2010-11 has already been spent or committed. The result, says Unison, is "eye-watering" cuts for the second half of the year.

Among the counties worst squeezed is Norfolk, where, says Unison, a 50% cut means that 65 full-time equivalent (FTE) jobs will go, centres will close and guidance instead will have to be given on the phone or the internet. Meanwhile, Northamptonshire faces a 40% cut in funding for the second half of 2010-11, with all 175 staff at risk of redundancy.

Unison is examining whether some councils may breach their statutory duty to provide adequate careers and personal advice. But proving any breach would entail a legal battle. "Any action would have to be taken by an individual – you'd have to sue the council to prove that you haven't been served properly," says John Richards, Unison's senior national manager.

Governments, of course, can always pass new laws that sweep away old obligations to provide services. Some professionals in the careers and youth services fear this is what will happen.

Little comfort is offered by the Local Government Association (LGA), which favours targeting money "more effectively" – focusing careers services on disadvantaged children while ensuring "better support" and information is available for the parents and teachers, "who most other youngsters first turn to for advice".

Careers advice for 16- to 18-year-olds need not be "under the Connexions brand", an LGA spokesman says. "Councils are looking to see how they provide this service in the most effective way, at the best value for money."

Since Connexions was dismissed by David Cameron as "not being a great success story ... not being very good value for money", the National Youth Agency (NYA) has announced the findings of a study it conducted for the LGA among 79 young people in London and south-west England, some of whom reported that Connexions advisers "don't respect a person's opinions and don't treat young people as an individual".

It was commonly felt Connexions focused on the vulnerable; but that if someone appeared to be "doing all right", they were left to "get on with it". Yet those who had close contact with Connexions were "very positive".

With further studies to start next month in the north-west, north-east, West Midlands and East Midlands, Fiona Blacke, the NYA's chief executive and a former head of Connexions, stands by the outcome of the survey, despite the small sample. "We feel this report provides a real insight," she says. "It would be grossly unfair if we represented their views in any other way."

However, the National Connexions Network (NCN) says these findings contradict other research, including a survey of more than 5,000 young people that found that over 90% of respondents were satisfied with what they received.

One battleground to save the Connexions service is Brighton, where 45 jobs could be at risk. Unison, with 4,000 members among Brighton council employees, has threatened a local ballot for strike action if redundancy notices are issued to personal advisers at Connexions.

Brighton MP Caroline Lucas has written to Eric Pickles, the communities secretary, saying cuts would "demolish years of relationships". "Connexions is far more than a careers service," she says. "The social impact seems to run counter to what the coalition government is trying to achieve. It's an economic nonsense as well – it will cost the state so much more to pick up the pieces down the line."

That view is shared by one Connexions adviser in Brighton who does not want to be named. Her patch covers two large deprived council estates. "If the cuts go through, it will be a disaster for young people and their families," she says.

For Geoff Wilson, 21, who dreams of a job tackling the city's rats and mice, a drop-in club run by Connexions was a turning-point. "I wasn't going anywhere with my life. They helped me make some goals and got me into college," he says. With an eye to his future, Wilson has since learned about gamekeeping and ferreting.

Through Connexions, Gemma Hickman, 19, found an apprenticeship with a Brighton plumbing firm after being unemployed for more than a year. "It's ridiculous they're getting cut," she says. "Connexions helped me when I was in an abusive relationship and made me feel better about myself."

Another Connexions user, Daniel Cappelli, 19, has written to every Brighton councillor protesting at the cuts. His advisers "have been the only positive adults in my life". "I had emotional issues, and issues with drugs and alcohol. Now I don't touch drugs and I'm working." Cappelli has started a college diploma in mechanical engineering and is employed at Sussex University on a heating installation project.

In Surrey, Unison says, Connexions advisers are grappling with a careers budget for schools that's been cut by 50%, leaving staff able only to give careers and course advice to those with statements of special educational needs. "We've been told we can't work with [other] vulnerable people in schools," says one adviser. "Responsibility has been put with schools, but a lot don't have careers teachers. A lot of kids come from very dysfunctional families. They [teachers] aren't trained for that."

She says tensions have arisen in some instances where voluntary-sector youth workers have been asked to plug the gap: "We're trained and qualified careers advisers. We've been losing our profession bit by bit, and in Surrey the trend is to cut us almost completely."